I think there's the fragility of a biplane to consider when training too, as wheels up landings WILL occur at some point, whilst the student pilot learns what the word "stall" actually means to him/her. With a biplane there's just more to break when you do lawn dart it.

Box trainers serve almost all student pilots well, and are built to take the occassional thump, and more importantly, be dead easy to patch up afterwards.

ChrisHays, it's great that you were able to fly a moderate model early on, but I don't think anyone will disagree that for every lucky person who manages to fly such a model as a beginner, several others will simply jack it all in.

I have to disagree that most instructors will teach on any model, it's my experience that this is simply not the case. Reactions vary, but ultimately most will refuse.